The present invention relates to a field effect transistor and a method for fabricating the same, and more particularly, it relates to a field effect transistor usable as a high-power radio-frequency transistor and a method for fabricating the same.
Group III-V nitride compound semiconductors, which are typified by gallium nitride (GaN), are represented by a general formula of InxAlyGa1-x-yN (wherein 0≦x≦1 and 0≦y≦1), have a wide band gap and a large breakdown field, and hence are regarded as promising materials for high-power electronic devices. In particular, in a heterojunction structure in which aluminum gallium nitride (AlGaN) is stacked on gallium nitride (GaN), a strong electric field is caused by polarization on the (0001) plane. Therefore, electrons are accumulated in a high density in the vicinity of the heterojunction interface in the GaN film, so as to generate the so-called two-dimensional electron gas (2DEG). Since the 2DEG is generated in the undoped GaN film, it is not affected by impurity scattering, and hence it exhibits high electron mobility. In addition, a GaN-based material exhibits high saturated drift velocity, and for example, in a high electric field region of approximately 1×105 V/cm, electronic velocity is twice or more as large as that of a GaAs-based material which prevails as a material for a radio-frequency transistor. Furthermore, the breakdown voltage of a GaN-based transistor can be easily increased owing to the high breakdown field of GaN. Therefore, the GaN-based material is expected to be applied to a high-power radio-frequency transistor.
In a heterojunction field effect transistor (HFET), in order to improve the radio-frequency characteristics, it is necessary to a reduce parasitic resistance, particularly, a parasitic resistance what is called a source resistance between an ohmic electrode and the 2DEG. Therefore, it is examined to reduce the source resistance, for example, by employing a recess structure in which a low-resistant capping layer is placed between an ohmic electrode and a underlying heterojunction structure. For example, it has been reported that the source resistance is reduced and the radio-frequency characteristics are improved by employing a multilayered structure for a capping layer, in which thin layers of GaN and AlGaN are alternately stacked (see, for example, T. Murata et al., IEEE Transactions on Electron Devices, 2005, vol. 52, pp. 1042-1047).
However, a conventional GaN-based transistors has the following problems: An ohmic electrode formed on a capping layer generally consists of a layered structure of titanium (Ti) and aluminum (Al). In order to reduce the contact resistance of the ohmic electrode which consists of the layered structure of Ti and Al, it is necessary to perform annealing designated as alloy processing after forming the electrode. Therefore, the process for forming the electrode is disadvantageously complicated. Also, the characteristics of the device are disadvantageously degraded because a semiconductor layer is thermally denatured or the electrode material is diffused into the semiconductor layer during the alloy processing.
Furthermore, in order to form an ohmic electrode with small contact resistance on an n-type semiconductor layer, it is necessary to use a metal with a small work function. On the other hand, in order to form a Schottky electrode with good Schottky characteristics, it is necessary to use a metal with a large work function. Therefore, a source electrode and a drain electrode should be made of a different material from that used for a gate electrode. For example, in general, a layered structure of titanium (Ti) and aluminum (Al) is used for a source electrode and a drain electrode with nickel (Ni) used for a gate electrode. Therefore, the gate electrode cannot be formed simultaneously with the source electrode and the drain electrode, which makes the process for forming the electrodes complicated.